Yellow vetch is a rather rare native plant of coastal shingle and bare open land. One of its strongholds is the Sussex Coast in places like Eastbourne, Shoreham Harbour and Pagham. Elsewhere it has been widely recorded as an alien.
The example above (identity confirmed by Paul Harmes, the East Sussex Flora Recorder) was brought to me from Barcombe by a friend who farms there. The opened flowers are white rather than yellow, though the unopened buds have a yellowish cast.
Away from the Sussex Coast, the Ouse Valley appears to be a route that the plant is taking inland. It has been recorded from Friston Forest and from several places in the Lewes area which lies a short distance to the south of Barcombe. Since its arrival on the farm there it has spread to a second field. It is described in the Atlas of British Flora as a "Submediterranean - subatlantic species." Maybe global warming is helping it spread northwards.